Sizeshifter

A very specific type of superpower. The character has the ability to change their size. Usually, this starts off one way, such as only being able to shrink or grow. Eventually, this starts to seem a little lame (unless the change comes with added Super Strength, or other Required Secondary Powers in order to offset that pesky Square-Cube Law), and they probably end up being able to do both with an upgrade of some sort, or a Hand Wave about training.

The three main characters in Ultimate Girls have the ability to grow to 100 feet tall.

Kirara of Inuyasha can alternate between her housecat-sized form and her battle-ready form, which is roughly the size of a horse.

A power of the Guyver Gigantic, growing into the massive Gigantic Exceed. Useful when one of your enemies turns into a Kaiju. It's implied that not only does it increase the Guyver's physical size but also increases the power of the Giga-Smasher and the Gravity Orb. To the point where Sho considered using gravity orb to create a small black hole (an extremely difficult and almost uncontrollable feat when previously done by Guyot) to be the less destructive option than the Giga-Smasher Exceed!

Mazinger Z: Several Mechanical Beasts had the ability to changing size. The first of them -Bicong O9- showed up as soon as episode 12.

Great Mazinger - Marquis Yanus -one of the Co-Dragons- could change size, although she -and her Humongous Mecha- became smaller instead of bigger (Justified since she was a spy and by dwarfing her mecha to human size she could blend between humans), so maybe it invers the trope.

Employed by the Akimichi clan in Naruto as part of their clan jutsu. Choza is the first one to turn into a giant onscreen and has spent most of the ninja war fighting in this form, and Choji uses it a couple of times as well.

Ant-Man and company (Black Goliath, Stature, Wasp, and Yellow Jacket) of The Avengers. Started out only being able to shrink. Later gained growth powers.

The original Ant-Man, Hank Pym, invented Pym Particles which enable this exact power, and allow anyone who can get their hands on them to use it. Said particles were used, among other things, to create a super-villain prison where all the villains were 6 inches tall. Needless to say, there was a breakout.

The Atom of the Justice League of America. Since the power comes from his belts, anyone who gets their hands on one can use it, much like the Pym Particles. As a consequence, there have been a few successors to the name, as well as an Evil Counterpart, Dwarfstar.

DC's original shrinking guy, Doll Man, had the not-too-impressive ability to shrink to about six inches... though he retains his full-sized weight and mass, resulting in a measure of super strength. There's a modern Doll Man, as well, but he's permanently tiny.

The Atom is an impressive version of this power set. Rather than gaining the ability to grow, he eventually gained the ability to shrink down to subatomic levels, opening up possibilities available to almost no one else. In addition, he can alter his molecular density, which enables him to glide on a low setting or have the equivalent strength of his regular size on a high one.

Another member of the Atom legacy, Atom Smasher of the Justice Society of America, has the ability to grow. Unlike the Pym Particles, this is accomplished by his bones and muscles actually breaking and reforming as he enlarges.

Colossal Boy and Shrinking Violet of The Legion of Super Heroes. In the '90s version of the team, Colossal Boy died and Violet gained his growing powers, rechristening herself LeViathan. In the '00s version, Colossal Boy lives and Vi has been renamed Atom Girl.

In the Threeboot ('00s) version, Colossal Boy prefers to be known as "Micro Lad", as his actual ability is to shrink to human size from his normal sixty-foot-tall size.

Among villains, one of the best known examples is Giganta from the DCU. However, she didn't develop this ability until she appeared in Superfriends, making the current version of her something of a Canon Immigrant.

Micro-Might from PS238. Shrinking means her body increases in density, increasing her strength and durability. However, if she gets too dense, she's unable to move. Also, since she isn't going to move fast like this, the Union of Justice tactics included having the Rockslidethrow her at the enemy once she's toughened enough.

In Wildguard, Jack Giant and Blaction Figure had growing and shrinking powers, respectively

Mighty Man, from Amazing Man Comics, has the ability to change his size.

Miss America, who was originally created for Military Comics but later became part of The DCU, has the power to shrink.

Justin Stewart of Power Rangers Turbo, a ten-year-old Child Prodigy who was the blue ranger, shifted to adult size when he morphed for some reason. He shifted back whenever he removed his helmet, oddly enough. It was almost like he could only maintain his shifted size in situations where he'd be played by a stunt double.

Perhaps needless to say, tabletop RPGs created to handle playing in the superhero genre usually include both powers. Fights between characters of massively disparate sizes can also seriously challenge a game's usual combat rules, which usually assume roughly human-sized opponents facing each other as the unspoken default and whose designers may not have stopped to consider how to address questions such as "can I climb/fly into my giant-sized opponent's ear and knock him out from there?"

Originally introduced in the 1st Edition Oriental Adventures sourcebook (and then seen again in Complete Arcane) are the Giant Size and Minute Form spells. Even a Squishy Wizard is going to hurt when she's hitting you with a tree.

Psionic characters have the Expansion power, which achieves the same effect. It has the added bonus of working on non-humanoids just as effectively as humanoids and even allowing the target to grow even more. There is, of course, an opposite that causes the target to reduce in size.

In Changeling: The Lost, there exists a Kith (sub-type) of the Ogres known as Gargantuans; their particular gift as Changelings is called Spurious Stature, which enables them to grow to enormous size for a brief time. Given the somewhat more subdued nature of the New World of Darkness, this is less extreme than most examples of this type, though a newly escaped Gargantuan can grow to the size of a gorilla or grizzly bear depending on their power. Supplementary material implies that their size-changing power is a consequence of their reduced power on Earth; supposedly, some Gargantuans remember being of perpetual giant-size in the domain of the Fae.

The multi-kith rules also make it possible for other sorts of Ogres to do this as well, or any possible Changeling, depending on what's house-ruled in.

Mario, of course. Firstly he had the Super Mushrooms that made him grow. Then in New Super Mario Bros. a Mini Mushroom was introduced which shrunk him to fit into tiny spaces and Walking on Water, as well as a Mega Mushroom which made him even bigger than with the Super Mushroom.

Although New Super Mario Bros. series seems to continue the adjustment first seen in Super Mario Bros. 2, that the "big" size is actually the standard; Mario and friends shrink when they get hit and then need to use a Super Mushroom to return to normal.

Bowser had occasionally became a giant in some games, and Rosalina actually has the ability to grow to the size of a planet!

Wario Land's Wario, in symmetry with his redder counterpart—but he also loses most of his hair when he shrinks.

Touhou: Suika Ibuki's most famous use of her density manipulation comes in the form of her 'Missing Purple Power' spellcard, which causes her to roughly triple her size. Fanon depictions sometimes show her becoming a literal 50-foot oni and beyond. What makes this even more noticeable is the fact that Suika is one of the shortest characters in the series at normal size. She also has the shrinking variation: She can generate mini-Suikas which are roughly 3 inches tall.

In Super Pac-Man, Pac-Man could eat a Super dot and temporarily become enormous. The super-sized Pac-Man was much faster and could pass through locked doors and ghosts without a problem.

Many characters from El Goonish Shive can do this, either by science or magic, because the whole genre of the comic is tranformation. But special mention goes to Rhoda who can change the size of herself or others from pocket-sized to basketball player-sized.

In the Whateley Universe, not only are there many people who can do this, but there is more than one superpower that will let you do it. Some Warpers can shrink or grow by using a warp displacement field about themselves so that they don't change but their relationship to the outside world is altered. Some Shapeshifters can shrink or grow also, and they have the Magic Pants problem.